The present invention relates to a honeycomb ceramics structure body which, for example, can attain a high collection efficiency with a low pressure loss and can be suitably used as a diesel particulate filter (DPF), and a method for producing the same.
In these years, the diesel particulate filters (DPFs) for collecting particulates discharged from diesel engines have been attracting attention, which are required to attain high collection efficiency with low pressure loss.
As DPFs, the honeycomb structure bodies made of cordierite have been conventionally used; in pursuit of such high collection efficiency with low pressure loss as described above, the improvement has hitherto been made in the honeycomb structure bodies as to the porosity and pore distribution thereof.
JP-A-9-77573 discloses a honeycomb structure having a specified the pore distribution on the surface of the partition walls with an enlarged porosity and an enlarged average pore size. JP-A-11-333293 describes honeycomb structure body having an enlarged porosity in addition to thin partition walls of a prescribed value or less.
In addition, JP-B-7-38930 discloses the production method for a honeycomb structure body having a high porosity by using a cordierite-forming raw material containing a talc powder and a silica powder each composed of coarser particles of a prescribed particle size or more. Japanese Patent No. 2726616 discloses a honeycomb structure body having a specified pore distribution and surface roughness in addition to a high porosity.
In the above described prior art, in order to increase the porosity, a cordierite-forming raw material is pulverized into coarse particles, graphite, wood powder, and a foaming agent are added as pore forming agents, or the like, but sufficiently satisfactory effects have not yet been obtained.
More specifically, when a cordierite-forming raw material is pulverized into coarse particles, the cordierite-forming reaction does not proceed to a sufficient extent, so that it is difficult to attain a low thermal expansion. When graphite is used as a pore forming agent, the following problems occur: the dielectric constant of a formed body with addition of graphite is decreased, so that it becomes difficult to perform a uniform drying of the formed body by the dielectric drying or the microwave drying with increase in addition amount of graphite. Furthermore, the firing period at the range from 800 to 1000xc2x0 C. is to be so elongated in the firing process that it is necessary to suppress the rapid combustion of the graphite.
Moreover, when the starches or wood powders are used as the pore forming agent, it is necessary to add a large amount of water in order to make the body for ceramics attain a prescribed hardness in the kneading process, so that the efficiency in the drying process becomes poor; and in the firing process the starches and wood powders rapidly burn between 200 and 400xc2x0 C. to release a large amount of heat, so that it is difficult to prevent the firing cracking. As above, in the prior art, it has been extremely difficult to increase the porosity beyond a prescribed value.
As a result of a diligent investigation performed in view of the above described problems in the prior art, the present inventors reached the present invention based on the findings that a very low pressure loss and a high collection efficiency can be attained when the porosity of the honeycomb structure body is increased to a prescribed value or more, and the total sum of the areas of the pores exposed on the partition wall surfaces is made to a prescribed value or more, with paying attention to the importance of the pore area on the partition wall surfaces with which surfaces the exhaust gas actually comes into contact and through which surfaces the exhaust gas passes.
In other words, according to the present invention, there is provided a honeycomb ceramics structure body having chemical composition of 42 to 56 wt % of SiO2, 30 to 45 wt % of Al2O3 and 12 to 16 wt % of MgO, and the crystalline phase mainly composed of cordierite, characterized in that said honeycomb ceramics structure body has a porosity of 55 to 65%, an average pore size of 15 to 30 xcexcm; and the total area of the pores exposed on the surfaces of the partition walls constituting the honeycomb ceramics structure body being 35% or more of the total area of the partition wall surfaces.
In the honeycomb ceramics structure body of the present invention, it is preferable that the total area of the pores exposed on the partition wall surfaces is 40% or more of the total area of the partition wall surfaces, and the average pore size is from 15 to 25 xcexcm. Furthermore, it is preferable that the partition wall thickness is 300 xcexcm or less. In addition, the permeability preferably is from 1.5 to 6 xcexcm2. It is also preferable that the coefficient of thermal expansion of the honeycomb ceramics structure body of the present invention between 40 and 800xc2x0 C. is 0.5xc3x9710xe2x88x926/xc2x0 C. or less.
The honeycomb ceramics structure body of the present invention can be suitably used as a diesel particulate filter (DPF) collecting the particulates discharged form a diesel engine.
In addition, according to the present invention, there is provided a method for producing a honeycomb ceramics structure body having chemical composition of 42 to 56 wt % of SiO2, 30 to 45 wt % of Al2O3 and 12 to 16 wt % of MgO, the crystalline phase mainly composed of cordierite, a porosity of 55 to 65%, an average pore size of 15 to 30 xcexcm; and the total area of the pores exposed on the surfaces of the partition walls constituting the honeycomb ceramics structure body being 35% or more of the total area of the partition wall surfaces, characterized in that 15 to 25 wt % of graphite and 5 to 15 wt % of a synthetic resin are added as a pore forming agent to a cordierite-forming raw material, the resultant is kneaded and molded into a honeycomb shape, and thus formed body is dried and fired to produce a honeycomb ceramics structure body.
In the above description, the synthetic resin is preferably any one of poly(ethylene terephthalate) (PET), poly(methyl methacrylate) (PMMA), and phenolic resin, or a combination thereof, and the average particle size of the raw material talc in the cordierite-forming raw material is preferably 50 xcexcm or less and the average particle size of the raw material silica is 60 xcexcm or less.